Can-ends are formed in a continuous process by presses that form the can-end and a transfer belt that transfers the can end blanks. The transfer belt supports the can ends during stamping and transports the can-ends away from the stamping station. Can-ends are typically made from aluminum and various aluminum alloys. Often, can-ends are made from a different alloy than the can body. Prior to forming, the metal stock needs to be cleaned, pretreated and over-coated with a protective coating in order to render the can-ends chemically resistant. Over-coats typically include vinyl, acrylic, epoxy, and polyester based paints, enamels, lacquers, and the like.
Cleaning and pretreatment compositions for aluminum and other metallic surfaces are typically aqueous acidic or alkaline compositions that may contain added chromium and fluoride. Furthermore, many of the compositions for treating metallic surfaces taught in the prior art contain hexavalent chromium or other inorganic oxidizing agents which are environmentally undesirable.
An improved composition and method of treating metallic surfaces which does not contain hexavalent chromium is provided in U.S. Pat. No. 5,897,716 (the '716 patent.) The '716 patent provides an aqueous metal-treating composition that includes a fluoroacid and a water soluble organic acid. The composition of the '716 patent also optionally includes a polymer composition, a pH adjusting component, an inorganic acid that contains fluorine, an antifoaming agent, and a component selected from the metals Ti, Zr, Hf, Al, Si, Ge, Sn, and B, and oxides, hydroxides, and carbonates of these metals. However, the '716 does not appreciate the inclusion of phosphates in a metal-treating composition for obtaining improved corrosion resistance.
Accordingly, there exists a need for metal treatments that do not contain undesirable components while providing improved corrosion resistance and adhesion to metal surfaces.